I just completed another 2 and a half cohorts for my Imperial Romans. I already had 4,5 cohorts done, for a new total of 7. 3 more and I complete my legion!
Even though I'd love to go ahead and work on the other 3 right away, I'll try to stay practical here and focus on what I need to actually start playing. 7 cohorts of legionaries is a good main force, now I need support for them. That means cavalry, auxiliary infantry and artillery. And then, of course, a rival army!